Will 2012 go down as the year that left the idea of nuclear energy expansion in the hot, dry dust?

Nuclear energy might be an important weapon in the battle against climate change, some scientists have argued, because it doesn't emit greenhouse gases. But separate of all the other issues with nuclear, that big plus would be moot if the plants couldn't operate, or became too inefficient, because of global warming.

In June, InsideClimate News reported on the findings of Dennis Lettenmaier, a researcher at the University of Washington. His study found that nuclear and other power plants will see a 4 to 16 percent drop in production between 2031 and 2060 due to climate-change-induced drought and heat.

The U.S. is getting plenty of both this year. And just Sunday, the Millstone nuclear plant in Waterford, Conn., had to shut down one of its two reactors because seawater was too warm to cool it. It was the first time in the plant's 37-year history that the water pulled from the Long Island Sound was too warm to use.

So the question becomes, is the future already here?

Heat records have been falling by the thousands since spring, and the U.S. Drought Monitor map showed last week that 62.46 percent of the nation is under moderate to exceptional drought conditions. That's down slightly from the peak of 63.86 percent last month, the highest percentage since the Drought Mitigation Center began producing the map in 2000. But the percentage of the country that is experiencing extreme to exceptional drought continued to rise and is now at 24.14 percent, almost a fourth of the country.

Much of the drought and unusual heat has been in areas that rely in part on nuclear plants: the upper Midwest, the Southeast and parts of New England.

When all of the nation’s 104 nuclear plants are fully operational, they supply about 20 percent of the energy generated in the United States. Those plants need water to operate, and in most cases, they need fresh water. There's not a lot of fresh water to go around in much of the nation this summer, and that is putting nuclear energy to the test.

It's also raising questions about how freshwater supplies should be managed in a world further taxed by climate change. Inevitably, there will be increased competition for water from a growing population, agriculture and the energy sector.

(Plants that use saltwater for cooling generally don't have the same issues, because they never have a shortage of water. But the shutdown at Millstone shows they can still be vulnerable to heat waves.)

About 40 percent of the nation's fresh water use goes toward energy generation, with nuclear energy considered a very water-intensive energy source.

Depending on whom you talk to, this simmering summer has highlighted a major drawback to nuclear energy, or it has demonstrated there's not too much to worry about.

"In terms of weather conditions, (the drought and heat) is not something that affects the operation of plants any more than we would normally see in a hot summer," Nuclear Regulatory Commission spokesman Scott Burnell said.

"In designing and building plants, they take into account the possibility of extreme drought. They can reasonably expect to take water in, even in extreme drought.

"The nut graph is, the plants continue operating."

Nuclear energy opponents say that's an oversimplification that glosses over this year's problems and the long-term vulnerabilities of the nuclear industry. They say this year is not the first, and won't be the last, that drought and heat waves force tough choices between environmental degradation and power shortages or expensive, stop-gap solutions.

"This has happened many times in the past, which makes us worry a lot," said David Kraft, director of the Nuclear Energy Information Service, a Chicago-based nonprofit committed to ending nuclear power. "In a climate-disrupted world, nuclear power plants are not reliable partners."

In late July, power generation from the nation’s nuclear plants fell to 93 percent of capacity, the lowest level since 2003. Not all of the drop was due to weather-related issues, but some of it was:

The Vermont Yankee plant near Brattleboro had to limit output four times in July because of low river flow and heat. At one point, production was reduced to 83% of capacity.

FristEnergy Corp's Perry 1 reactor in Ohio dropped production in late July to 95 percent of capacity because of above-average temperatures.

Operators of the Braidwood, Ill., nuclear plant 60 miles southwest of Chicago sought and were granted a waiver from the Environmental Protection Agency to raise the temperature of a cooling pond to 102 degrees - 2 degrees above the established limit. The pond holds water cycled through the plant for cooling and then discharged. If the plant had not received the waiver, it would have had to scale back production in the middle of an intense heat wave.

Kraft said the nuclear plants' operating difficulties are part of a recurring pattern. In the summer of 1988, drought, high temperatures and low river volumes forced Commonwealth Edison to reduce power by 30 percent or shut down, in some cases, at the Dresden and Quad Cities plants in Illinois.

"That was the first wake-up call that plants would be vulnerable in a climate-disrupted world," Kraft said.

There have been many more instances since:

Europe, summer, 2003. During the heat wave that killed more than 30,000 people, France, Germany and Spain had to choose between allowing reactors to exceed design standards and thermal discharge limits and shutting down reactors. Spain shut down its reactors, while France and Germany allowed some to operate and shut down others.

Illinois, summer of 2005. EPA and state officials considered easing thermal discharge standards because of drought, but a break in the weather made it unnecessary.

Illinois, Minn., July 29 to Aug. 2, 2006. The Prairie Island (Minn.) plant had to reduce output by 54 percent. The Quad Cities, Dresden and Monticello plants in Illinois also cut power to moderate water discharge temperatures.

Michigan, July 30, 2006. The Donald C. Cook reactors in Michigan were shut down during a severe heat wave because temperatures in a containment building exceeded the regulatory limit of 120 degrees.

Southeast U.S, Aug. 5-12, 2008. The Tennessee Valley Authority lost a third of nuclear capacity due to drought conditions. All three Browns Ferry reactors in Alabama were idled to prevent overheating of the Tennessee River.

France, July 2009. France had to purchase power from England because almost a third of its nuclear generating capacity was lost when it had to cut production to avoid exceeding thermal discharge limits.

Southeast U.S., July, August 2011. The TVA reduced power at Browns Ferry to stay within discharge limits. At one point, all three of the reactors cut output to about 50 percent. Had the plant been operating at full capacity, the downstream temperature on the Tennessee River would have exceeded the 90-degree limit.

NRC spokesman Burnell said plants have had to dial back production at times, but he wasn’t aware of rolling blackouts in any of those instances.

Why the water matters

Drought and heat affect the water needed to cool nuclear plants in three main ways.

Most plants are built along lakes or rivers because of the ready supply of water to cool the plant. But if water levels drop below the plants' intake pipes, they can't suck in the water. In some cases, the pipes can be lowered, but that is often expensive and also risks pulling in sediment that could damage the plant.

Lake or river water can also be unusable if it's too warm to cool the plant. Other times, the warm water can be used, but the plants operate far more efficiently with cooler water.

The third problem that crops up in heat waves is with the outflow from the plant. If the water body is already warm because of low levels or hot days, the outflow could raise the downstream temperature above accepted levels. That has happened repeatedly in the U.S. and in Europe. If a plant isn't shut down in those situations, the hot discharge can cause algae blooms, reduce dissolved oxygen in the water and threaten aquatic life.

"From our perspective, this has been a problem for years, and it's only getting worse," said Wendy Wilson, director of rivers, energy and climate for the River Network, which aims to protect the nation's freshwater resources. "It gets worse every time we have a drought like this.

"We have terrible thermal pollution problems in this country, and the result is dead and dying rivers. Nobody's managing the system. We're all just praying for rain."

Besides the Tennessee River in Alabama, Wilson said, there are energy-related overheating problems on the Oconee in Georgia, the Rio Grande in Texas, and in Lake Erie.

Burnell said Browns Ferry in Alabama is looking at modifying its cooling mechanisms so it can return discharged water without raising the temperature of the Tennessee River.

But similar modifications have been tried for decades at other plants, Kraft said, and consumers always end up paying for it. If adding a new technology involves a shut down, a plant can lose $1 million a day in revenue. And if the utility is forced to buy energy from another source, it is usually at much higher rates.

"Every time you go to the newest adaptation, you're adding costs," Kraft said. "Every new trick like this costs hundreds of millions of dollars. That's got to end up on somebody's balance sheet."

Robert Krier has been writing about weather and climate issues for U-T San Diego for 13 years. InsideClimate News is a non-profit, non-partisan news organization that covers energy and climate news.